A light emitting diode (LED) is a semiconductor diode that radiates light. The light emitting diode converts electric energy into ultraviolet light or visible light.
Conventionally, LED lamps including light emitting diodes are widely used.
For example, when a light emitting chip is formed by using a substrate such as a transparent substrate and a light emitting material such as GaP, GaAsP, GaAlAs, GaN, or InGaAlP and sealed by transparent resin, an LED lamp that radiates visible light is obtained. When the light emitting material is fixed on an upper surface of a printed board or a metal lead and sealed by a resin case molded to a number or a character, an LED lamp of a display type that radiates visible light is obtained.
A replacement frequency of the LED lamp is reduced and extension of life thereof is possible because the light emitting diode as a semiconductor element having long life and high reliability is used. Therefore, LED lamps are widely used as components of display devices of a portable communication apparatus, a personal computer peripheral apparatus, an OA apparatus, a home appliance, audio equipment, vehicles such as an automobile, a signal light, a switch, a light source display plate for backlight, and the like.
If various kinds of phosphor powder are included in a front surface of the light emitting chip, the sealing resin, or the resin case, the LED lamp can adjust a color of radiated light. Specifically, if the light emitting chip and the phosphor powder that absorbs light radiated from the light emitting chip and emits light in a predetermined wavelength band are used in combination, it is possible to cause the LED lamp to emit light in a visible light domain and white light corresponding to applications according to an action of the light radiated from the light emitting chip and the light radiated from the phosphor powder.
Among the LED lamps, in particular, a white light emitting LED lamp is currently increasingly used in a backlight of a cellular phone and a vehicle-mounted lamp. In future, it is expected that the white light emitting LED lamp substantially grows as a substitute for a fluorescent lamp. For example, a RoHS regulation (a harmful chemical substance regulation) of EU prohibits use of mercury. Therefore, it is predicted that the white light emitting LED lamp that does not use mercury replaces the conventional fluorescent lamp that uses mercury.
As the white light emitting LED lamp, for example, a white light emitting LED lamp of a type obtained by combining a blue light emitting diode chip and a yellow light emitting phosphor (YAG) and further combining a red phosphor if necessary (hereinafter referred to as “type 1 of the white light emitting LED lamp”) and a white light emitting LED lamp of a type obtained by combining an ultraviolet ray or violet light emitting diode chip and blue, yellow, and red phosphors (hereinafter referred to as “type 2 of the white light emitting LED lamp”) are currently used or studied.
Among these white light emitting LED lamps, the type 1 of the white light emitting LED lamp is preferred and used because the type 1 has higher luminance than the type 2.
As a material of the yellow phosphor used in the type 1 of the white light emitting LED lamp, a cerium activated yttrium aluminate phosphor (YAG), a cerium activated terbium aluminate phosphor (TAG), an alkali earth silicate phosphor, and the like are known.
Among these materials, the YAG and the alkali earth silicate phosphor are already used in a fluorescent lamp for a backlight of a cellular phone, a flying spot scanner, and the like. However, there is a problem in that luminance of the YAG and the alkali earth silicate phosphor is insufficient for used in illumination, a headlamp of a car, and the like.
In recent years, since color perception of users is improved, the LED lamp is required to have high-definition reproducibility of a subtle tone and uniformity of an external appearance.
In response to the request, various proposals have been made to increase luminance and to give high color rendering properties and the uniformity of an external appearance to the white light emitting LED lamp.
For example, Patent Document 1 (International publication No. 2003/021691) discloses a white light emitting LED lamp including the alkali earth silicate phosphor and having a high luminous flux. This white light emitting LED lamp has a high luminous flux equivalent to that of the white light emitting LED lamp including the YAG and has little color unevenness.    Patent Document 1: International Publication No. 2003/021691